The present disclosure relates generally to covert communication systems to send signals to intended users in such a way so as to minimize the ability of an unintended receiver from detecting or intercepting the signal.
In the field of covert communications, transmissions may be typically identified under one of three categories: 1. Low Probability of Detection (LPD); 2. Low Probability of Interception (LPI); and 3. Low Probability of Exploitation (LPE).
LPD is the characteristic of a signal that prevents an unauthorized receiver from detecting the presence of the signal. This non-detection is commonly achieved by transmitting a signal at a low enough power density to hide in or under the noise floor.
LPI is the characteristic of a signal that prevents an unauthorized receiver from capturing a previously detected signal. This non-capturing is commonly achieved by spreading the signal's energy over a wider bandwidth than is necessary for ordinary communications, thus making direct reception of the signal impractical by an unauthorized receiver without knowledge of the spreading mechanism.
LPE is the characteristic of a signal that prevents a captured signal from being exploited by an unauthorized receiver. Non-exploitation is commonly achieved through the use of data encryption.
In one way, a signal is made to be covert, with LPD and LPI characteristics, by spreading the original non-covert signal with a pseudo-random noise (PN) sequence. The original signal is multiplied by a waveform of chips, wherein each chip is a pulse that is either positive or negative representing the binary value (1 or 0) of the PN sequence. Because the chip rate is higher than that of the original signal's data rate, the effect is that the resulting signal's energy is spread over a wider bandwidth than the original signal alone. Since the energy is constant, the relative height of the spectrum is reduced allowing the signal to “hide” in or under the noise.
Many methods exist for exploiting the structure of a pseudo-random noise (PN) sequence to better detect and de-spread the signal. Once the de-spreading is complete, ordinary signal reception is possible leaving the information in the signal vulnerable for exploitation. A layer of encryption can protect the main information content of the signal but often these algorithms require header information and other structures that can still be exploited by an unintended receiver without requiring sophisticated methods for breaking the encryption scheme.
Attempts to make signals covert may also utilize chaotic signal generation. A chaotic signal is generated from a system that displays properties of chaos. Although there is no generally agreed upon standard definition of chaos, properties that are required include deterministic behavior (as opposed to random behavior) that is sensitive to initial conditions such that the long term evolution of the system cannot be predicted. Essentially, despite being deterministic, a chaotic system's future is nearly impossible to predict. A result of this property is that if the characteristics (e.g. coefficients) are different between two chaotic systems of identical structure then the two systems will diverge rapidly even when the difference is small. This makes characterizing a chaotic system from an output chaotic signal extremely challenging. But this also makes a chaotic system useful for LPI systems, because after applying the chaotic signal to the desired message it is nearly impossible to regenerate chaos without knowing ahead of time the exact characteristics of the original chaotic system. These characteristics become the “security key.”
What is needed is a device and process to effectively communicate signals covertly having LPD and LPI characteristics without being corrupted by noise during transmission. The device and process would need to be used in realistic field environments, and not for example requiring a clock for synchronization between transmitter and receiver. What is also needed is for synchronization to not have to occur after the transmission of every bit, which ultimately affects data throughput. What is also needed is that the device be not susceptible to a statistical analysis by an unintended receiver to predict the information transmitted. What is also needed in the field is the capability to separate the device, or hardware, from the “security key” so that physical components do not need to be replaced to change the characteristics of the chaos.